Abstract

Conspecific sperm precedence (CSP) has been identified as an important post-copulatory, pre-zygotic mechanism that can act to reduce gene flow between populations. The evolution of CSP is thought to have arisen as a by-product of male and female coevolution in response to intraspecific post-copulatory sexual selection. However, little is known about the mechanisms that generate CSP. When Cnllosobruchus
subinnotatus females copulate with both C. subir~notanis and Callosobruchus maculatus males, regardless of mating order, the majority of eggs are fertilized by conspecific sperm. The low numbcr of heterospecific fertilizations does not result from general differences in the viability of sperm in the female reproductive tract, as heterospecific sperm fertilized equivalent numbers of eggs as conspecific sperm in the absence of sperm competition. Instead, CSP results from disadvantages to heterospecific sperm that are manifest only when in competition with conspecific sperm. CSP in C. subinnotatus appears to result from two, not
mutually exclusive, mechanisms. First, conspecific sperm are better able to displace heterospecific sperm from female storage. Second, conspecific sperm achieve disproportionately higher numbers of fertilizations
relative to their proportional representation in the fertilization set. Thus, we provide evidence of differential
sperm use from the female spermatheca.